Senator Siewert said the Greens had the Parliamentary Budget Office cost their policy and examine its funding plan.
The increase would cost $5.6 billion over the forward estimates for Newstart and $1.7 billion for Youth Allowance.
The Greens would fund the plan with a new tax on wealthy Australians and by increasing the mining tax to a 40 per cent tax on super profits, as originally proposed under former prime minister Kevin Rudd.
They have proposed a new 50 per cent tax rate on millionaires which they say could raise $791 million over three years.
An estimated $7.3 billion, out of an estimated $33 billion from boosting the mining tax, would be allocated to the welfare proposal.
Senator Siewert said the $50 weekly boost to welfare payments was modest and long overdue.
People could stop skipping meals and not be squeezed over rent and bill payments.
It would also improve job prospects because welfare receipts will be able to put money towards haircuts, internet connections, computers or interview clothes, Senator Siewert said.
"People are saying it's not as much as we need, we'll still be living from hand to mouth," she told AAP.
"But it's a step in the right direction and will ease the pressure somewhat."
She hopes the bill will be debated in the Senate ahead of the federal budget in May or the September election.